The present invention relates to a negative-feedback amplifier and, in particular, relates to an apparatus and method for controlling a phase of a demodulated baseband signal.
In the field of radio communication using Multi Channel Access, there is a problem related to the relationship between power efficiency and linearity, that is, if the power efficiency of a power amplifier located at the last stage of a transmitter is increased, the nonlinear distortion of the power amplifier becomes large.
Because of the above relationship, in the case of high density digital transmission, for example, in the case of sending a signal modulated by a linear modulation method such as 16 QAM (Quadrature Amplitude Modulation), deterioration of the transmission spectrum caused by nonlinearity of an amplifier can not be avoided.
For compensating for this deterioration, an apparatus and method to restrain distortion using a negative-feedback circuit has been developed. Specifically, the method seeks to obtain a high open-loop gain for a signal in a high frequency band. Such a method is described in "Reduction Of Spurious Emission From Radio Transmission By Means Of Modulation Feedback" by V. Petrovic, IEEE Conf. Radio Spectrum Conservation Tech., September 1993.
In this method, an output signal of a power amplifier is demodulated and the demodulated signal is fed back in the form of a baseband signal.
FIG. 3 illustrates a block diagram of a negative-feedback amplifier using this method.
X1(t), Y1(t) are components of a two-dimensional baseband signal.
31 is a subtracter for subtracting a two-dimensional demodulated baseband signal (described later) from the two-dimensional baseband signals which are generated from transmission data in an external circuit (not shown in this figure) and independent of each other.
32 is a low pass filter for limiting the frequency band of an output of the subtracter 31.
33 is a modulator for QA (Quadrature Amplitude)-modulating a two-dimensional carrier signal (described later) with a two-dimensional signal output from the low pass filter 32 and for outputting a modulated signal.
34 is a carrier generator for outputting a two-dimensional carrier signal for modulation and demodulation.
35 is an amplifier for amplifying the modulated signals output from the modulator 33.
36 is a phase shifter for adjusting the phase of an output signal output from the amplifier 35.
37 is a demodulator for demodulating a two-dimensional demodulated baseband signal from the output signal of the phase shifter 36 and the two-dimensional carrier signal and for outputting a result.
Next, an operation of the negative-feedback apparatus described above is explained.
First, a two-dimensional baseband signal X1(t), Y1(t) is generated from transmission data in an external circuit for QA-modulation, and is supplied to the subtracter 31. The subtracter 31 subtracts a two-dimensional demodulated baseband signal that is output from the demodulator 37 from the two-dimensional baseband signal X1(t), Y1(t).
The low pass filter 32 that receives this subtraction result limits the frequency band of the feedback circuit.
Next, the carrier generator 34 outputs a two-dimensional carrier signal cos(.omega.c.times.t), sin(.omega.c.times.t), which are orthogonal to each other, wherein, .omega.c is the angular frequency of the carrier.
The modulator 33 QA-modulates the two-dimensional carrier signal cos(.omega.c.times.t), sin(.omega.c.times.t) with the two-dimensional signal X3(t), Y3(t), and outputs a modulated signal Z(t). EQU Z(t)=X3(t).times.cos (.omega.c.times.t)+X3(t).times.sin (.omega.c.times.t)(1)
The modulated signal Z(t) is amplified by the amplifier 35 and sent out via an antenna (not shown in the figure).
Next, the phase shifter 36 that receives a part of the output signal output from the amplifier 35 adjusts the phase of the output signal of the amplifier 35 for preventing deterioration of the open-loop gain caused by circuit delay. The demodulator 37 demodulates the two-dimensional demodulated baseband signal from an output of this phase shifter (36) and the two-dimensional carrier signal cos(.omega.c.times.t), sin(.omega.c.times.t) output from the carrier generator 34 and outputs a result.
Here, supposing that a gain of the amplifier 35 is G, a delay from input to output in the amplifier 35 is .delta., the ratio of output of the amplifier 35 that is distributed to the phase shifter 36 is R and the phase shifter 36 does not adjust the phase, the two-dimensional demodulated baseband signal P(t), Q(t) is represented by the equations (2) and (3). EQU P(t)=G.times.R.times.{X3(t-.delta.).times.cos (.omega.c.times..delta.)-Y3(t-.delta.).times.sin (.omega.c.times..delta.)}(2) EQU Q(t)=G.times.R.times.{Y3(t-.delta.).times.cos (.omega.c.times..delta.)+X3(t-.delta.).times.sin (.omega.c.times..delta.)}(3)
However, for the two-dimensional demodulated baseband signal P(t), Q(t) in the equations (2) and (3), there may be a condition, cos(.omega.c.times..delta.).noteq.1.
In the two-dimensional demodulated baseband signal P(t), Q(t) at this time, the respective components of second terms Y3(t-.delta.).times.sin(.omega.c.times..delta.) and X3(t-.delta.).times.sin(.omega.c.times..delta.) are leaked signals against the respective components of the first terms X3 (t-.delta.).times.cos(.omega.c.times..delta.) and Y3(t-.delta.).times.cos(.omega.c.times..delta.). That is, the following equations are not realized.
Y3(t-.delta.).times.sin(.omega.c.times..delta.)=0. PA1 X3(t-.delta.).times.sin(.omega.c.times..delta.)=0. PA1 a detecting means for detecting a phase error of the demodulated baseband signal caused by a delay in the amplifier for each change of a sending frequency of the carrier; and PA1 a first phase controlling means for controlling the phase of an output signal of the amplifier so as to extinguish a phase error of the demodulated baseband signal based on a detected phase error of the demodulated baseband signal. PA1 a reference signal generating means for generating a reference signal; PA1 a subtracting means for subtracting the demodulated baseband signal from the reference signal and outputting a subtraction signal; and PA1 a phase difference detecting means for detecting a phase difference between the reference signal and the subtraction signal. PA1 a phase shift quantity calculating means for calculating a phase shift quantity of an output signal of the amplifier so as to extinguish a phase error of the demodulated baseband signal based on a detected phase difference; and PA1 a phase shifting means for receiving a phase shift quantity calculated by the phase shift quantity calculating means and shifting the phase of an output signal of the amplifier based on this phase shift quantity. PA1 a phase shift quantity calculating means for calculating a phase shift quantity of the carrier so as to extinguish a phase error of the demodulated baseband signal based on a detected phase difference; and PA1 a phase shifting means for receiving a phase shift quantity calculated by the phase shift quantity calculating means and shifting the phase of the carrier based on this phase shift quantity. PA1 a reference signal generating means for generating a reference signal; PA1 a subtracting means for subtracting the demodulated baseband signal from the reference signal and outputting a subtraction signal; PA1 a phase difference detecting means for detecting a phase difference between the reference signal and the subtraction signal; PA1 a phase shift quantity calculating means for calculating a phase shift quantity of an output signal of the amplifier so as to extinguish a phase error of the demodulated baseband signal based on a detected phase difference; and PA1 a phase shifting means for receiving a phase shift quantity calculated by the phase shift quantity calculating means and shifting the phase of an output signal of the amplifier based on this phase shift quantity. PA1 an indicating means for outputting an indication signal to indicate a start of correction for a phase error of the demodulated baseband signal; and PA1 a selecting means for outputting a reference signal supplied from the reference signal generating means when having received the indication signal. PA1 a reference signal generating means for generating a reference signal; PA1 a subtracting means for subtracting the demodulated baseband signal from the reference signal and outputting a subtraction signal; PA1 a phase difference detecting means for detecting a phase difference between the reference signal and the subtraction signal; PA1 a phase shift quantity calculating means for calculating a phase shift quantity of the carrier so as to extinguish a phase error of the demodulated baseband signal based on a detected phase difference; and PA1 a phase shifting means for receiving a phase shift quantity calculated by the phase shift quantity calculating means and shifting the phase of the carrier based on this phase shift quantity. PA1 an indicating means for outputting an indication signal to indicate a start of correction for a phase error of the demodulated baseband signal; and PA1 a selecting means for outputting a reference signal supplied from the reference signal generating means when having received the indication signal. PA1 an indicating means for outputting a correction indication signal to indicate a start of correction and a change of a sending frequency; PA1 a carrier generating means for generating a predetermined carrier by changing the sending frequency when having received the correction indication signal; PA1 a reference signal generating means for generating a reference signal; PA1 a selecting means for receiving a baseband signal and the reference signal and in a normal operation, outputting the baseband signal, and outputting the reference signal when having received the correction indication signal; PA1 a subtracting means for subtracting the demodulated baseband signal from an output of the selector; PA1 a low pass filtering means for receiving an output signal of the subtracter and limiting the frequency band of this output signal; PA1 a modulating means for modulating the carrier with an output signal of the low pass filtering means and outputting a modulated signal; PA1 an amplifying means for amplifying the modulated signal; PA1 a phase difference detecting means for detecting a phase difference between an output signal of the low pass filtering means and the reference signal; PA1 a phase shifting means for calculating a phase shift quantity of an output signal of the amplifying means so as to extinguish a phase error of the demodulated baseband signal based on a detected phase difference detected by the phase difference detecting means when having received the indication signal; and PA1 a demodulating means for demodulating the demodulated baseband signal from the carrier and an output signal of the amplifier that has been phase-shifted by the phase sifting means. PA1 an indicating means for outputting a correction indication signal to indicate a start of correction and a change of a sending frequency; PA1 a carrier generating means for generating a predetermined carrier by changing the sending frequency when having received the correction indication signal; PA1 a reference signal generating means for generating a reference signal; PA1 a selecting means for receiving a baseband signal and the reference signal and in a normal operation, outputting the baseband signal, and outputting the reference signal when having received the correction indication signal; PA1 a subtracting means for subtracting the demodulated baseband signal from an output of the selector; PA1 a low pass filtering means for receiving an output signal of the subtracter and limiting a frequency band of this output signal; PA1 a modulating means for modulating the carrier with an output signal of the low pass filtering means; PA1 an amplifying means for amplifying the modulated signal; PA1 a phase difference detecting means for detecting a phase difference between an output signal of the low pass filtering means and the reference signal; PA1 a phase shifting means for calculating a phase shift quantity of the carrier so as to extinguish a phase error of the demodulated baseband signal based on a detected phase difference detected by the phase difference detecting means when having received the indication signal; and PA1 a demodulating means for demodulating the demodulated baseband signal from the carrier that has been phase-shifted by the phase sifting means and an output signal of the amplifier. PA1 demodulating a demodulated baseband signal from a carrier and an output of an amplifier; PA1 detecting a phase error of the demodulated baseband signal caused by a delay in the amplifier for each change of a sending frequency of the carrier; and PA1 correcting the phase error of the demodulated baseband signal based on a detected phase error of the demodulated baseband signal. PA1 subtracting the demodulated baseband signal from a reference signal and outputting a subtraction signal; and PA1 detecting a phase difference between the reference signal and the subtraction signal.
Under this condition, the distortion improvement effect can not be obtained sufficiently because open-loop gain is equivalently reduced, or occasionally positive feedback may occur and the circuit oscillates.
Then, to avoid such deterioration, the phase is adjusted in the phase shifter 36.
Here, supposing that a delay in the phase shifter 36 is .tau., and the phaser shifter 36 adjusts the phase for .omega.c.times..tau., the two-dimensional demodulated baseband signal P(t), Q(t) output from the demodulator 37 is represented by the following equations. EQU P(t)=G.times.R.times.[X3(t-.delta.).times.cos {.omega.c.times.(.delta.+.tau.)}-Y3(t-.delta.).times.sin {.omega.c.times.(.delta.+.tau.)}] (4) EQU Q(t)=G.times.R.times.[Y3(t-.delta.).times.cos {.omega.c.times.(.delta.+.tau.)}+X3(t-.delta.).times.sin {.omega.c.times.(.delta.+.tau.)}] (5)
Therefore, by determining .omega.c.times..tau. so as to realize the following equation (6), both the components of the second terms of the equations (4) and (5), Y3(t-.delta.).times.sin{.omega.c.times.(.delta.+.tau.)} and X3(t-.delta.).times.sin{.omega.c.times.(.delta.+.tau.)} becomes 0, only components of the first term remain. Thus, the above problem can be avoided. EQU .omega.c.times.(.delta.+.tau.)=2.times.n.times..pi.(n is an integer)(6)
Thus, a negative-feedback amplifier that feeds back a signal in the form of a baseband signal can be configured.
In the negative-feedback amplifier described above, however, when the delay .delta. of the amplifier 35 has frequency characteristics against a sending frequency (in the carrier angular frequency .omega.c=2.times..pi..times.f, this f is the sending frequency), the equation (6) may not be realized if the sending frequency changes because the quantity .omega.c.times..pi. is fixed at a predetermined value in the above negative-feedback amplifier.
As a result, for a system that changes the sending frequency, the open-loop gain and distortion improvement effect in the above negative-feedback amplifier deteriorate.